Drag is a mechanical force generated by a solid object moving through a fluid (a liquid or gas). When the fluid is a gas like air, it is called aerodynamic drag (or air resistance). When the fluid is a liquid like water, it is called hydrodynamic drag. In one example, sources of aerodynamic drag is the skin friction between the molecules of the air and the solid surface of a wing or fuselage (such as those found in an aircraft). In another example, sources of aerodynamic drag is the skin friction between the molecules of the air and the solid surface of rotor blades (such as those found in a wind turbine). Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both solid and gas. For the solid, a smooth, waxed surface produces less skin friction than a roughened surface. For the gas, the magnitude depends on the viscosity of the air and the relative magnitude of the viscous forces to the motion of the flow, expressed as the Reynolds number. Along the solid surface, a boundary layer of low energy flow is generated and the magnitude of the skin friction depends on conditions in the boundary layer.